We knew T-Mobile was gearing up for some kind of big sale on September 24 based on a leak, but it has been made official today. T-Mobile will be putting all of its smartphones, tablets and mobile broadband devices on sale as part of a Value Plan promotion. It all sounds great on paper, but in reality, it’s hard to get excited about this “sale.”

You may remember that not too long ago, T-Mobile broke its contract plans into two categories – Classic and Value. The biggest difference between the two deals are monthly installment payments for Value plans. Value customers have to play the subsidized price for a phone, then pay as much as $15 a month for 20 months to make up the difference between the subsidized price and the retail price of the device.

In terms of this one-day sale, new and existing customers can participate. T-Mobile provided prices of some of the devices that will be on sale such as the HTC Sensation 4G and the T-Mobile myTouch 4G Slide. Each of these devices will cost $99.99 after a $100 mail-in rebate. They also include a monthly payment of $15. Let’s break this deal down using the Sensation 4G.

If you participate in this promotion, you’ll pay $199.99 for the Sensation 4G in the store. You’ll also pay $15 a month for 20 months. That comes to a grand total of $300 in monthly payments. If you subtract the $100 rebate, you’ll end up paying $200 when the 20 months is over. $200 what T-Mobile charges for the Sensation 4G on its Classic plans.

So why would anyone take part in this sale? They could be sucked in by the allure of a rebate, or they may like the idea of not being required to have a data plan. On Classic plans, T-Mobile requires a data plan if you purchase a smartphone. Data plans are available, but optional with Value plans and are cheaper as well.